This invention relates to transient fuel control apparatus for an internal combustion engine and particularly for such an engine provided with above the throttle fuel injection. With such a system, air flow to the engine is varied by the vehicle operator through a throttle valve in a throttle body and fuel is injected through one or more injectors mounted upstream from the throttle in response to a control signal derived from a pair of engine operating parameters such as engine speed and a load indicating factor such as the pressure below the throttle in the throttle body or intake manifold. Especially when supplemented by a closed loop air/fuel ratio control system, such apparatus can provide good control of air/fuel ratio at a substantially stoichiometric ratio to provide efficient reduction of undesirable emissions in a three-way catalytic converter under substantially steady engine operating conditions.
If the engine throttle is moved substantially, however, and particularly if it is moved quickly, transient conditions are created which can cause temporary but significant deviations from the desired air/fuel ratio and thus momentarily but significantly decrease converter efficiency for one or more of the undesirable emissions. It is well known, in general, that extra fuel must be added to the engine as the throttle opens and must be withheld from the engine as the throttle closes if a substantially constant air/fuel ratio is to be maintained during these transient conditions. However, the precise amount and timing of the fuel addition or subtraction required has proved to be difficult to attain.
One approach of the prior art is shown in the U.S. Pat. to Endo No. 3,759,231, in which a first order lag filtered value of manifold pressure is subtracted from the present value of manifold pressure to generate a difference signal varying with the rate of change of manifold pressure relative to the filtered value; and this signal determines the amount of extra fuel to be added. However, in the U.S. Pat. Nos. to Shinoda et al 3,719,176 and 3,842,811, the patentees point out that there is a delay time involved in manifold pressure responsive systems; and they themselves describe a system in which transient fuel control is triggered by a significant rate of change in throttle position. Since the transient correction is generally required over a longer period of time than that of the throttle movement itself, Shinoda et al provides an initial extra amount of fuel dependent in quantity on the size of the rate of change of throttle movement with additional pulses following in quantities that decrease over time. While it is true that detection of throttle movement provides a faster response to the transient condition than does the detection of a change in manifold pressure, this type of system has proved relatively difficult to calibrate exactly for the many different sizes of transient correction and initial engine conditions. For example, the engine may be calibrated to respond correctly to a wide open throttle acceleration at low engine speed with an initial amount of extra fuel and rate of decay that is inappropriate for the same wide open throttle acceleration initiated when the engine is at a significantly higher engine speed. In practice, this approach tends to involve increasing numbers of provisions for one factor or special case after another with different system gains and amounts until the total system is extremely complex; and even then there generally appears to be an engine operating condition for which it could be improved.